Bipolar transistors (BJTs) are commonly used in semiconductor devices especially for high speed operation and large drive current applications. A double polysilicon BJT 10 is shown in FIG. 1. The area for the BJT 10 is isolated by field oxides 12. The collector 14 is a lightly doped epitaxial layer of one conductivity type and the base region is formed by doped regions 16 and 18 of the opposite conductivity type. Doped region 16 is called the intrinsic base region and region 18 is the extrinsic base region. The extrinsic base region 18 provides an area for connecting to the base region. The base electrode 20 comprises a first doped polysilicon layer. The emitter region 22 is a doped region of the same conductivity type as the collector and is located within the intrinsic base region 16. The emitter electrode 24 is accomplished with a second doped polysilicon layer. Oxide region 26 and base-emitter spacers 28 isolate the emitter electrode 24 from the base electrode 20. Double polysilicon BJTs have the further advantage of lower base resistance and reduced extrinsic capacitances over single polysilicon BJTs. However, this advantage is gained by accepting additional process complexities such as those associated with the etching of polysilicon from the active device areas and the out diffusion of a base link-up doping region from a highly doped polysilicon diffusion source.
Another prior art double polysilicon BJT 50 is shown in FIG. 2. A base link-up region 52 is used to contact the intrinsic base region. Base link-up region 52 is formed by the diffusion of dopant from polysilicon spacers 54. An insulator region 56 separates the base polysilicon from the collector region. Advantages of this approach include the elimination of extrinsic capacitance and reduced device area. However, this approach still has the process complexity of having to etch polysilicon directly from the silicon active area.
The advantages of the double polysilicon BJT must currently be balanced against the process complexities described above. Accordingly, there is a need for a method of forming a BJT that reduces these process complexities.